U.S. Pat. Nos. 3,904,102 and 4,032,033 to T. Y. Chu et al. disclose vapor condensation mass soldering apparatus of a type which may be used for soldering articles, such as solder-bearing leads, to contact pads on a substrate circuit device. In this apparatus a primary liquid having a boiling point at atmospheric pressure at least equal to the temperature at which the soldering is to be performed, is continuously boiled to establish a body of hot saturated primary vapor having a density greater than that of air at atmospheric pressure. A "blanket" in the form of a body of secondary vapor, having a density intermediate that of the primary vapor and the atmosphere, is produced by boiling a secondary liquid and is floated on the body of primary vapor to reduce losses thereof to the atmosphere.
In use of the above-mentioned vapor condensation soldering apparatus, the articles to be soldered generally are prefluxed and then passed through the body of secondary vapor into the body of primary vapor. Primary vapor then condenses on the articles, with the latent heat of vaporization of the condensing primary vapor heating the articles to the temperature required for the soldering operation. After completion of the soldering operation, the articles are withdrawn from the body of primary vapor through the blanketing body of secondary vapor.
Escape of the secondary vapor from the above-mentioned apparatus is minimized by mounting secondary cooling or condensing coils about the inner periphery of the apparatus adjacent an upper end thereof, to cause condensation of the secondary vapor adjacent an upper end of the secondary vapor blanket. Similarly, primary cooling or condensing coils are mounted internally about the inner periphery of the apparatus at an intermediate level to cause condensation of the primary vapor at an upper end of the body of primary vapor. The condensate from the condensation of the secondary and primary vapors is collected for recycling in suitable troughs extending about the inner periphery of the apparatus below the secondary and primary condensing coils, respectively.
Vapor condensation soldering apparatus as above described frequently requires cleaning or replacement of the condenser coils, particularly where large quantities of rosin flux are used to facilitate the soldering operation. In this regard, rosin flux causes a carbonaceous deposit to build up on the surfaces of immersion heaters used to produce the condensation soldering vapors, causing undesirable overheating of the heaters and thermal degradation of the primary and secondary liquids. The thermal degradation of the primary liquid can produce a build up of hydroflouric acid in the system, causing etching of metal parts, such as the condenser coils. Similarly, the thermal degradation of the secondary liquid, in the presence of organic matter, such as the soldering flux, can produce a buildup of hydrochloric acid, with similar effects. Under very high acid conditions in larger condensation soldering facilities, the thermal degradation of the primary liquid also can cause the deposition of a white crystalline material (perflourobutylamide) on cool surfaces of the apparatus, such as the condenser coils.
In the past, cleaning or replacement of the internally mounted condenser coils of the vapor condensation soldering apparatus was a difficult and time-consuming procedure because of the manner in which the condenser coils were mounted in the apparatus. As a result, the cleaning or replacement of the condenser coils required that the apparatus be taken out of manufacturing service for extended periods of time. Further, since the condenser coils were mounted on interior portions of the apparatus, cleaning or replacement of the condenser coils required that a technician work inside the apparatus, which also was undesirable because of the possible exposure to residual toxic gases (e.g., perflouroisobutylene or phosgene) which may have been generated in the apparatus during operation and which may still remain in the apparatus after it has been shut down.
Accordingly, a primary purpose of this invention is to provide a new and improved vapor condensation soldering apparatus in which cleaning or replacement of condenser coils in the apparatus is greatly facilitated, and in which the need for a technician to work inside the apparatus for this purpose is essentially eliminated.